PROJECT SUMMARY/ABSTRACT The brain is able to form learned associations between stimuli and the rewards or punishments they predict, which then guide appropriate behavioral responses to maximize survival. However, responses to environmental stimuli are severely altered in neurological disorders such as anxiety, depression, and post- traumatic stress disorder, leading to damaging behavioral outcomes. Understanding the neural circuits which mediate learned associations and motivated behavior is therefore of crucial importance both from a basic science and human health perspective. The amygdalostriatal transition zone (ASt) has circuit connectivity which suggests a role in these processes, but the function of this region is almost completely unknown. The central hypothesis of this proposal is that the ASt acts as a parallel circuit to the amygdala to mediate associative learning and behavioral responses. In the mentored phase (K99) experiments, a combination of optogenetics, in vivo electrophysiology and advanced computational analysis will be used to identify subpopulations of neurons in the ASt that encode stimulus information and specific behaviors. The independent phase (R00) experiments will determine the contribution of inputs from the thalamus, cortex and lateral amygdala to ASt activity encoding cue information, and determine whether synapses from these projections onto ASt neurons are strengthened in associative learning. Preliminary data for this study show that 1) genetically distinct populations of ASt neurons have opposing responses to cues predicting rewards or punishments, and 2) optogenetic inhibition of one of these populations results in a striking reduction in conditioned fear responses. This suggests that the ASt may indeed play a critical role in behavioral responses traditionally attributed to the amygdala. A successful outcome of this proposal could therefore result in a major conceptual revision of current models of the circuit mechanisms underlying associative learning. The study will also provide new insight into the long-standing open question of how significant behavioral responses to conditioned stimuli still persist even following bilateral loss-of-function manipulations targeting the amygdala. This will not only increase our fundamental knowledge of the circuits underlying motivated behavior, but could also identify ASt circuits as vital new targets of interest for neurological disorders where normal behavioral responses to stimuli are disrupted. All proposed research will be conducted Dr. Fergil Mills in the lab of Dr. Kay Tye at the Salk Institute for Biological Studies, which is fully equipped for the proposed experiments. The proposal also includes a comprehensive training plan to facilitate Dr. Mills's career development, and will prepare him to direct an innovative research program as an independent investigator studying neural circuit mechanisms underlying normal and pathological behaviors.